The present invention relates to digital radio systems, and more specifically, to performing synchronization as part of the processing of a received signal in a radiocommunication system.
Radiocommunication systems involve the transmission of information over an air interface, for example by modulating a carrier frequency with that information. Upon reception, a receiver attempts to accurately extract the information from the received signal by performing an appropriate demodulation technique. However, in order to demodulate a received signal, it is first necessary to synchronize timing between the transmitter and the receiver. For example, clocking differences between the transmitter and the receiver provide for differences in bit timing. Moreover, in some radiocommunication systems, information is transmitted in bursts, sometimes referred to as "frames". In these types of systems it is also desirable to locate the beginning of a frame, so that information relevant to a particular receiver is isolated and demodulated.
Unfortunately, there exist many challenges associated with synchronizing to a received signal. For example, although the receiver may be tuned to an assigned frequency on which its intended signal has been transmitted, Doppler shifting may result in a large frequency offset between the frequency to which the receiver is tuned and the actual frequency of the desired information signal when it reaches the receiver after having travelled through the air interface. Moreover, the crystal oscillator used in the receiver is only accurate to within a certain number of parts per million, which may introduce an additional frequency offset.
In addition to an unknown frequency offset, a receiver must also cope with unknown phase accuracy, i.e., the receiver does not know the difference between the phase of the signal generated by its synthesizer at power-on and the phase of the received signal. Thus, the receiver faces at least three challenges in synchronizing to the received signal: unknown timing, unknown frequency offset and unknown phase.
Despite these challenges, performance objectives established for today's receivers are remarkably high. For example, most receiver designs require that synchronization almost always (e.g., 96% of the time) be acquired during the first frame in a burst. This performance objective is even more daunting in the arena of satellite communication systems, where Doppler effects can be relatively great, power constraints require acceptance of a relatively low signal-to-noise ratio and the frequency channels can be relatively narrow. This latter characteristic makes the frequency offset described above even more significant, since it then becomes entirely possible that a desired information signal has been frequency shifted to the center frequency of an adjacent channel.
Due to the importance of synchronization, and its impact on demodulation, the literature is replete with various discussions of these problems. For example, the impact of mobile-cellular standards on modulator-demodulator (MODEM) selection and design is addressed in an article by Kamilo Feher entitled "MODEMS for Emerging Digital Cellular-Mobile Radio System" (IEEE Transactions on Vehicular Technology, Vol. 40, No. 2, May 1991). This article discusses various modulation techniques that are used in emerging second generation radio systems. As this article points out, most system standards do not dictate the demodulation architecture. Manufacturing companies may use coherent, differential, or discriminator techniques for signal demodulation.
The above article focuses primarily on II/4-QPSK modulated signals. Feher claims that large frequency shifts render an offset-QPSK modulated signal unsuitable for low bit-rate communication systems. The present invention, however, overcomes this problem.
A particular demodulation technique is described in a report by Gardner entitled "Demodulator Reference Recovery Techniques Suited for Digital Implementation" (ESA Final Technical Report 1989, ESTEC Contract No. 6847/86/NL/DG). Gardner suggests using a maximum likelihood approach to estimate frequency error, and then a phase error detector to correct for phase.
Another demodulation technique is described in an article by J. Ahmad et al. entitled "DSP Implementation of a Preambleless All-Digital OQPSK Demodulator for Maritime and Mobile Data Communications" (IEEE, 1993, pp. 4/1-5). According to Ahmad, phase lock loops (PLLs), which are generally employed for coherent detection, have inadequate pull-in range for the initial carrier frequency. Ahmad suggests using a dual filter discriminator to estimate frequency offset, an AFC loop to correct frequency error, and a second order split loop for phase error detection.
However, among other drawbacks, neither of these schemes are considered by Applicants to achieve sufficiently accurate synchronization at a high enough first frame success rate in the face of low signal-to-noise ratios. Therefore, it would be advantageous to provide new techniques for synchronizing to a received information signal that overcomes these drawbacks.